What should I do?

So my gf and I were mad at each other for some time a few weeks ago. During this time, she denied me of all forms of sex and it was driving me insane because it had been a while since we got down. We went to a house party together around this time and all our friends were there and the party was running pretty late so she told me she was tired and wants to go home. We came together and was ready to drive her back but because she wanted to be petty she got an Uber home. I was still having fun and was like there's no need for me to leave anymore. Everyone could tell we had issues. Long story, short my gf's best friend came to talk to me and one thing led to another and we fucked. My gf and I are back on good terms now and I still haven't told her about what happened. She also still hangs out with her best friend regularly so i know she didn't tell her either. Anytime she comes over she's sneaking in dirty looks at me like we should do it again and tbh I want to. I also don't want to leave my gf so I was gonna propose the idea of a threesome with her since she has talked about wanting to do one before and I know she'd enjoy it. Should I tell her what happened or see how this plays?